Uses new httpclient package from grafana-plugin-sdk-go introduced
via grafana/grafana-plugin-sdk-go#328.
Replaces the GetHTTPClient, GetTransport, GetTLSConfig methods defined
on DataSource model.
Longer-term the goal is to migrate core HTTP backend data sources to use the
SDK contracts and using httpclient.Provider for creating HTTP clients and such.
Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>